The present invention involves drilling bits and more specifically involves a rolling cutter drill bit comprising a bit body threaded for connection to a drill string and having a lower machined surface for receiving one or more rolling cutter assemblies which are affixed thereto by means such as electron beam welding.
The conventional drill bits in use today generally comprise three similar lug sections each having an integral bearing shaft projecting downwardly at an angle therefrom. The three lug sections have bearing surfaces machined on the shafts for receiving rolling cutter assemblies. After machining operations are completed on the bearing shafts, the three lugs are placed in a welding fixture and welded longitudinally to form a single drill bit structure. Afterwards, the upper end of the joined structure is threaded for engagement in a drill string connector. Rolling cutters are located on the bearing shafts with appropriate bearings located therein.
Prior to assembling the multiple lug sections of the prior art devices a required number of complex machining and hardening procedures must be applied to each lug in order that the necessary close tolerances in the bearing structures be achieved. These operations generally include accurately securing each lug in a grinding fixture and machining bearing surfaces thereon to extremely tight specifications having very close tolerances. The bearing shaft extending downwardly at an angle from each lug must be machined in multiple steps because of the varied number of bearing surfaces on the shaft.
Due to the length and awkward shape of each lug, the grinding procedures for forming the particular bearing surfaces are complex and difficult. Because of the concentric nature of the bearing surfaces on the main radial bearing portion of the journal and the pilot pin portion of the journal, very close tolerances must be maintained to reduce eccentricities between these two bearing surfaces. This is difficult to achieve and very expensive, partially because of the large awkward shape of the lug on which the machining operations are performed.
After machining the various bearing surfaces on each of three different lugs, the lugs must then be aligned properly to achieve the desired gage, which refers to the effective cutting diameter of the bit, then the three lugs are combined into a single drill bit structure by welding their common abutting edges together. The bit must then be machined to form threads at the upper end for connection to the drill string and this operation must also be very accurate to provide proper alignment and rotation of the cutters and the drill bit with respect to the rotational axis of the drill string.
A further disadvantage of the prior art bit construction is that, in addition to extensive hardening and heat treating, stronger and tougher alloys must sometimes be utilized in the construction of the bearing area on the bit lugs. Unfortunately, because the lugs are shaped in integral sections, these expensive alloys have to be used throughout the entire lug structure. Because of the prohibitive expense of many desirable alloys, they cannot be used in the prior art devices.
Another distinct disadvantage in the prior art devices is that the gage of the finished drill bit is difficult to maintain within specified tolerances because of their methods of construction. Likewise, each different size of drill bit requires entirely different sized and shaped lugs. Therefore, an 81/2 inch nominal bit requires a different lug than an 83/4 inch nominal bit. Also, achieving the actual desired effective cutting diameter is difficult because of the inaccuracy of welding the forged lug members into a single bit structure.
Other prior art devices utilize a welded structure wherein the entire lug is formed in a normal manner but without a bearing shaft projection thereon. Consequently, a cylindrical opening is machined through the downward end of the lug for receiving a separate bearing shaft which may then be pressed in place and/or welded. While this device may offer some advantages over the normal single piece lug construction of the other prior art devices, it does not offer the benefits of the present invention wherein the gage dimension may be accurately controlled to very tight tolerances and may even be varied within certain ranges to obtain a different size bit utilizing the same structural components. Likewise, the main body cannot be made in a single section but must be welded from three sections as in the other prior art bits. An example of the prior art device utilizing a pressed-in bearing shaft is disclosed in U.S. Pat. No. 4,043,411.
The present invention overcomes all of these disadvantages by providing a drill bit construction utilizing a single upper bit body which may be manufactured in a single section and which has a lower machined abutment surface generally normal to the central rotational axis of the drill bit. To this machined abutment surface a plurality of extension lugs having matching machined surfaces on the top thereof and rolling cutters mounted thereon are attached by means such as welding.
Each extension lug has a downwardly and inwardly extending bearing shaft on which are located machined radial and axial bearing surfaces. The machined abutment surfaces on the extension lugs are arranged so that they are generally normal to the axis of rotation of the drill bit. Location of each extension lug in its proper position along the bit body may be achieved by using location dowels set into recesses in the surface of the bit body and adapted to engage matching dowel recesses machined in the abutment surface of each extension lug.
Thus, the drill bit of this invention may utilize a single upper bit body which can be cast or machined according to conventional techniques from an inexpensive suitable metal such as cast iron, steel, or other inexpensive alloy. The smaller, more compact extension lugs may be formed of a more exotic, wear resistant material of a more expensive alloy.
The present invention is also advantageous in that the effective cutting diameter or gage can be more easily controlled by controlling the location of the compact extension lugs on the abutment surface of the bit body. Likewise, a standardization of manufacturing products can be accomplished by the use of single size bit bodies for varying size bits by sliding the extension lugs radially inward and outward before welding to obtain the differing gage sizes.
Furthermore, the present invention is advantageous in that the machining of bearing surfaces on the bearing shafts projecting from the extension lugs is accomplished far easier because of the small size and regular shape of the extension lugs. Thus, the present invention reduces the extensive machining and metal hardening requirements of the prior art devices, eliminates the need for three large lugs to be joined into a single drill bit body, and allows the use of more exotic alloys in the critical areas in the drill bit construction. The present invention also allows greater control of dimensions such as bearing eccentricities and gage dimensions.